The invention relates to an imaging device incorporating a single crystal semiconductor wafer and more particularly to a device having improved performance at about 65 degrees centigrade.
Imaging devices such as silicon vidicons and silicon intensifier tubes employ sensing elements or targets consisting of single crystal semiconductor wafers. The operation of such sensing elements in these devices is well known in the art. Such an imaging device is described in U.S. Pat. No. 4,232,245 issued to Savoye et al. on Nov. 4, 1980 and incorporated by reference herein for the purpose of disclosure. This patent describes an imaging device having a silicon target with reduced blooming.
The target element in the Savoye et al. patent comprises a wafer of semiconducting material doped to have an N- type conductivity, with one of the major surfaces of the wafer being selectively doped to have a large plurality of P- type conductivity regions, respective ones of the regions forming junction diode with the substrate thereunder. The substrate is maintained at a potential which is positive with respect to a scanning electron beam so that as the P- type conductivity regions are bombarded with electrons they become reversed biased. In the reversed biased state, each of the junction diodes stores an electric charge of electrons derived from the beam and maintains that charge at least until it is scanned again. The reverse bias maintained on the junction diodes in the target element and the charges stored in the diodes create a "depletion region" in the substrate under each diode that also extends between adjacent diodes. These depletion regions are characterised by a shortages of majority carriers (electrons in the case of an N- type conductivity material) and by an electron field across the depletion region.
In normal operation, the surface of the target opposite the aforementioned diodes, which is hereinafter referred to as the well-side of the target, receives light from an image. Photons striking the target cause the creation of electron-hole pairs to be generated in the substrate. A substantial number of holes, which are thus generated, reach the diode opposite the point of photon impact where they combine with and hence eliminate a corresponding number of stored electrons.
The holes generated in the substrate migrate through the body of the target until they reach the depletion region, where they are rapidly swept into the nearest diode by the electric field existing across the depletion region. In this manner a charge pattern is created in the diodes corresponding to the image striking the well-side of the target, with each of the diodes having its stored charge reduced by an amount corresponding to the time integral of the light striking the corresponding spot on the well-side of the target.
One problem associated with the operation of such silicon targets is commonly referred to as "blooming". "Blooming" occurs when the silicon target is exposed to a relatively high intensity point source of light in a relatively low light level background. Under these conditions, the number of holes generated as a result of the point light source is greater than the charge storing capacity of the corresponding diodes in the target. As a result, when less than all of the generated holes have discharged the diodes directly opposite the point of light impact, the remaining holes spread laterally through the target substrate and are swept into the nearest adjacent diodes, tending to discharge them as well. This lateral spreading of holes has the tendency of discharging too many diodes and is manifested by a relatively large, bright image having a size that increases with the intensity of the point light source. The Savoye et al. patent reduces blooming by forming a potential barrier with a controlled energy level configuration spaced from the well-side of the target. This potential barrier in normal operation allows a limited number of holes to penetrate to the well-side of the target and then recombine, thereby maximizing the sensitivity of the device by permitting the greater majority of excited minority carriers, or holes, to diffuse towards the charge storage region of the wafer. However, in the case of the generation of excess carriers by overexcitation at localized regions (normally associated with the blooming conditions previously described) the excess carriers accumulate at and overcome the potential barrier. These excess carriers are swept to the well-side of the target where they quickly recombine due to the substantially increased recombination velocity along that surface, thereby avoiding lateral diffusion to neighboring diodes in the charge storage region of the target.
A second limitation of the junction diode target is that it tends to have a leakage current, also called a dark current, which discharges the reverse biased diodes even in the absence of light. Principally, leakage currents are caused by the flow of minority carriers (e.g., holes in an N- type substrate) from the substrate into the P- type conductivity diode regions where they recombine with, and hence eliminate, electrons stored therein. Such leakage currents are known to be caused by the generation of electron-hole pairs in the body of the target element at particular sites called "generation-recombination centers" which occur where there is a defect or impurity in the semiconductor substrate, where there is a transition between differently doped areas in the material, and where the substrate interfaces with another material. The leakage current herein described may be either voltage dependent or temperature dependent.
The voltage dependency of the dark current is discussed in U.S. Pat. No. 3,717,790 issued to Dalton et al. on Feb. 20, 1973. In the Dalton et al. patent, voltage dependent leakage is viewed as being generated on the diode side of the target, specifically at the silicon-silicon oxide interface and the patent only addresses a solution to diode side leakage. Likewise, U.S. Pat. No. 3,828,232 issued to Horiike et al. on Aug. 6, 1974 and U.S. Pat. No. 3,883,769 issued to Finnila on May 13, 1975 limit their investigation of target related problems to the diode side of the target. The aforementioned dark current leakage is generally a phenomena which affects a large portion or all of the diodes so that a generally uniform increase in target dark current occurs. Certain types of leakage, however, are manifest as localized disturbances and are frequently seen as cosmetic defects, e.g., intense white spots. Such defects occur when locally generated holes discharge one or more adjacent diodes without affecting other diodes in the array. Such defects may be either voltage related and/or temperature related.
In many camera tube applications it is necessary to operate the camera tube at elevated temperatures in the neighborhood of 65.degree. C. Unfortunately, many tubes which operate acceptably at lower operating temperatures (within the range of 23.degree. C. to 40.degree. C.) fail at an elevated temperature approaching 65.degree. C. Such a failure occurs from the generation of temperature dependent defects. These defects are believed to arise principally, but not exclusively, from local sites at the well-side surface of the silicon target and are thus not effectively treated by the leakage reduction techniques described in the aforementioned patents which consider only diode side leakage.